The present invention concerns a method of synchronising the analogue display of a timepiece whose movement is fitted with an electronic time base. This time base is fitted with a quartz oscillator well known to those skilled in the art. The analogue display includes an analogue indicator coupled in rotation to a first watch movement wheel. Within the scope of the present invention, this first wheel is associated with a proximity sensor and has at least one aperture in its plate that is preferably made of an electrically conductive material. The proximity sensor is arranged for detecting the aperture of the first wheel and more specifically for determining the position of a reference geometrical semi-axis defined by said aperture.
In particular, the analogue indicator is a time indicator.
The principle of detection of the semi-axis defined by an aperture provided in the wheel coupled to an analogue indicator is disclosed in particular in EP Patent No. 0 952 426 (corresponding to U.S. Pat. No. 6,330,207). The arrangement of the proximity sensor relative to the wheel that is associated therewith and the operation thereof will be briefly described hereinafter with reference to FIGS. 1 to 3.
Watch movement 2 comprises a toothed wheel 4 with a toothing 6 defined on the outer edge of plate 8 of said wheel. Wheel 4 further comprises an arbour 10 defining a geometrical axis of rotation 12. Watch movement 2 is associated with an analogue display comprising a rotating indicator 14, in particular a hand, fixedly mounted on arbour 10. Indicator 14 can be used to indicate the hour, minute or any other useful information required. Within the scope of the present invention, indicator 14 belongs to an analogue display of the current time, the object being to synchronise this indicator with an electronic time base of the watch movement. Indicator is secured to plate 8 in rotation.
Plate 8 has a circular aperture 16. It will be noted that the contour of this aperture can be different, particularly rectangular. An inductive sensor 20, formed of an integrated circuit 22 and a flat spiral coil 24 arranged on one face of the integrated circuit, is arranged relative to plate 8 such that flat coil 24 is at least partially below aperture 16 in at least one given angular position of wheel 4. It will be noted that the coil can be connected to the electronic circuit without being arranged thereon.
Preferably, as shown in FIG. 1, central magnetic axis 26 of the spiral forming flat coil 24 passes substantially through the geometrical centre 28 of aperture 16 in a given angular position of wheel 4. Aperture 16 defines a reference geometrical semi-axis REF of wheel 4. This semi-axis starts from geometrical axis 12 and passes through the centre 28 of circular aperture 16. Inductive sensor 20, which forms a proximity sensor, is arranged for determining the position of reference semi-axis REF.
Sensor 20 includes for example an electronic circuit of the type described in EP Patent No. 0 746 100 (corresponding to U.S. Pat. No. 4,240,528) with a differential relaxation oscillator. This type of sensor provides an alternating signal whose frequency varies as a function of the proximity of electrically conductive materials relative to the detection coil. Aperture 16 generates a variation in material in proximity to coil 24 when wheel 4 rotates. In certain cases, a sensor having a different electronic architecture based on a harmonic oscillator can be provided.
As shown schematically in FIG. 3, sensor 20 provides a measurement signal whose points of measurement 32 obtained as a function of the angular position α of reference semi-axis REF define a graph 30 that is substantially symmetrical in relation to a geometrical axis 34. The angular position of this axis 34 corresponds to an angle αREF of wheel 4 when its reference semi-axis REF intersects central magnetic axis 26 of flat coil 24. Enough measurements will be taken to obtain a density of points of measurement 32 allowing the negative slope and the positive slope of graph 30 to be differentiated, so that the position of the axis of symmetry 34 of graph 30 can be determined.
After having carried out the measurements necessary to determine the evolution of the sensor measurement signal when the aperture passes above the sensor coil, electronic means for processing the measurement results determine angle αREF and thus the corresponding position of hand 14.
It will be noted that the dimensions of coil 24 are preferably of the order of 1 mm2. When the diameter of the wheel with the aperture for detecting its position is relatively small, the aperture then preferably has a surface area defining an annular sector whose radial dimension is less than the tangential dimension, the radial dimension being then less than the corresponding dimension of the detection coil. In this latter case, the measurements for setting up graph 30 and determining the position of axis of symmetry 34 require wheel 4 to travel an angular distance of approximately 90°. In the most favourable case shown in FIGS. 1 and 3, the angular distance necessary is already of the order of 60°. These values are minimum values corresponding to the case in which the position of indicator 14 substantially corresponds to the theoretical position given by the electronic time base of the watch movement. If the position of the indicator associated with the detection wheel is located, particularly due to shocks received by the timepiece, in a relatively far position from the aforementioned theoretical position, the angular distance necessary for detection of reference semi-axis REF of the wheel will be greater than the values given hereinbefore.